Determining a property of a formation material

ABSTRACT

Method for determining a property of a formation material in the course of a jet drilling operation, wherein a fluid jet is blasted with erosive power into impingement with the formation material, the method comprising postulating a relationship between the erosive power and a removal rate of formation material as a result of the impingement of the fluid jet with the erosive power on said formation material, the relationship comprising a parameter related to at least one property of the formation material; determining removal rates of formation material for at least two settings of erosive power; and determining the at least one property of the formation material from the relationship and the determined removal rates in dependence on erosive power.

The invention is related to determining a property of a formation material in the course of a jet drilling operation, in particular into a subsurface earth formation.

According to a known approach, properties of a formation material can be derived by analyzing cuttings which result from a conventional mechanical drilling action. Furthermore, it is known to apply downhole evaluation measurements like gamma-ray and neutron density logs. Properties like permeability and porosity can be derived from such measurements. As much as these properties can be of general interest to the geologist, for the process of mechanical drilling itself they are normally not considered important parameters as such.

Alternatively to a conventional mechanical cutting drill bit a fluid jet drill head can employed, which directs a fluid jet with erosive power into impingement with the borehole wall. Preferably a fluid mixture including a quantity of abrasive particles is employed. Such jet drilling is particularly well suited for making boreholes with a small diameter. In contrast to mechanical cutting drilling methods, no or minimum weight on bit is necessary for drilling.

A jet drill system and method of making a hole in an object is for example disclosed in WO-A-2005/005767. The known system comprises an excavating tool, herein also referred to as abrasive jet drill head, mounted on a lower end of a drill string that is inserted from the surface into a hole in a subterranean earth formation. The drill string is provided with a longitudinal passage for transporting a drilling fluid mixture comprising abrasive particles to the drill head. The drill head comprises jet means arranged to generate an abrasive jet in a jetting direction into impingement with the earth formation in an impingement area. The abrasive jet contains magnetic abrasive particles (steel shot). A recirculation system is provided, which captures abrasive particles from the return stream to surface, after erosive impingement, by means of a magnet, and re-mixes the abrasive particles at a mixing location with the mixture received via the drill string. The magnet is arranged as a rotatable conveyor, attracting particles to be recycled and conveying them towards a mixing location with fresh fluid from surface. In the known system directional drilling is achieved by a modulation means in form of a controllable drive means for the conveyor, which is arranged so as to modulate the recirculation rate, and in this way the quantity of particles in the abrasive jet at the jet means is modulated. When the abrasive jet is moved along a trajectory in the hole, in particular in a rotating motion, the amount of erosion in each impingement area along the trajectory can be selectively varied, and directional control is achieved. Reference is also made in this regard to other known abrasive jet drill systems and methods as described for example in WO 00/66872, WO 2002/034653, WO 2005/005766, WO2008/119821, WO 2008/113843, WO 2008/113844.

Jet drilling, with or without abrasive particles, differs in many ways from mechanical drilling. A major difference is that jet drilling results in rock cuttings which are typically much smaller than the cuttings which result from mechanical drilling such as using rollercone or PDC drilling bits with cutters. The cuttings are predominantly fines which can hardly be distinguished with the bare eye, and larger chips are scarce.

WO 2009/099945 discloses a particle impact drilling system and method. It is disclosed that the impactors may be selected based upon physical factors, for example one or more rock properties of the formation being excavated. It is also disclosed that a rate of penetration can be optimized by determining an excavation parameter, and by adjusting one or more controllable variables accordingly. Excavation parameters are parameters relating to the excavation method used.

US 2006/0016622 discloses a method and system for excavating a subterranean formation using a high-velocity fluid with solid material impactors, wherein also excavation parameters are monitored, and at least one excavation parameter is altered accordingly.

There is a need in the art for a new method of determining a property of a formation material, in particular in the course of drilling a hole by jet drilling, such as abrasive jet drilling.

The present invention provides a method for determining a property of a formation material in the course of a jet drilling operation, wherein a fluid jet is blasted with erosive power into impingement with the formation material, the method comprising the steps of:

-   -   postulating a relationship between the erosive power and a         removal rate of formation material as a result of the         impingement of the fluid jet with the erosive power on said         formation material, the relationship comprising a parameter         related to at least one property of the formation material;     -   determining removal rates of formation material for at least two         settings of erosive power; and     -   determining the at least one property of the formation material         from the relationship and the determined removal rates in         dependence on erosive power.

Applicant has realized that in jet drilling, in contrast to mechanical drilling, properties such as permeability and porosity of the formation material do play an important role, and that this can be used with advantage. In prior art, such as in WO 2009/099945, US 2006/0016622, or GB 2 221 762, it has not been recognized that a material property, which is independent of the excavation method used, can be determined in this way.

Jet drilling with a high pressure fluid jet is for example very sensitive to rock permeability and porosity. A high pressure fluid jet erodes the rock material by impingement on the rock material. Above a certain bit or nozzle pressure drop threshold, the pores in the rock are blown up by the high velocity of the fluid jet. Porous and permeable formation materials such as sandstones or limestones can be jetted away easier than non-porous and non-permeable materials such as shale. This allows to obtain a direct indication of formation material parameters, without the need for drill cuttings analysis or separate downhole measurement tools.

In some preferred embodiments, the fluid jet is an abrasive fluid jet comprising abrasive particles. In this case, even more information on formation material parameters can be obtained, since abrasive particles interact differently with the formation material than a high pressure jet of fluid without abrasives. Abrasive particles exert a destructive effect on the rock due to the kinetic impact thereof. The volume removal rate caused by this effect increases with the kinetic energy of the abrasive particles, and with the abrasive particle concentration times the flow rate (nozzle pressure drop). There is no threshold to this effect, and mechanically soft rock is jetted away easier than hard rock. At the same time, there is the impact of the high velocity jet that erodes based on permeability and porosity of the rock. By varying the erosive power of the jet it can e.g. be detected whether a soft impermeable formation is drilled, a porous medium strength rock, or a hard and tight rock.

Suitably when using an abrasive jet, the concentration of abrasive particles in the fluid jet is different between the at least two settings of erosive power. The concentration of like particles such as steel shot in e.g. an aqueous liquid can conveniently be defined in terms of volume % of the fluid mixture, but in any event and independent of the precise constitution of abrasives in terms of wt % of the fluid mixture. It is ultimately the weight of particles impinging on a certain area of the formation material that is relevant for the kinetic energy. So two, or optionally more, settings of weight concentration of abrasive particles give insight into the dependence of volume removal rate of the formation material from the kinetic energy of abrasive particles.

Alternatively or in addition, at least one of a fluid pressure, pressure drop over a jet nozzle, or velocity of jetted fluid, is different between the at least two settings of erosive power. These parameters relate to fluid velocity, which has an influence on the erosive power of the liquid component of the jet alone, but also on the kinetic energy of abrasive particles when present.

In one embodiment, a classification of hardness of the formation material is obtained. Such classification can be a relative, qualitative or semi-quantitative parameter, such as relatively soft versus relatively hard, or classifying a plurality of formation materials encountered in the course of drilling a hole into the subsurface.

Alternatively or in addition, the method can comprise determining one or more of a porosity, a permeability, and/or a tensile strength. Tensile strength is a measure of hardness.

In particular embodiments the at least one property can comprises a rock type. This can e.g. be the case when a characteristic combination of other parameters is found, that can be found in a database; or can be compared with a with a calibration or comparison drilling operation in a known rock type; or when auxiliary information is available such as from an analysis of drill cuttings.

The postulated relationship can be qualitative, semi-quantitative or quantitative. In particular embodiment it includes the erosive power of hydraulic liquid and the erosive power of abrasive particles, and the at least two settings of erosive power are selected such that the effect of changing only erosive power of hydraulic liquid and/or only erosive power of abrasive particles can be determined individually, i.e. the influence from both effects can be separated in the analysis of the measurements.

In certain embodiments at last one of a threshold pressure and an increase of removal rate per increase of hydraulic pressure above a threshold pressure is determined for hydraulic liquid jetting, i.e. for the effect of the fluid or liquid component alone even if abrasives are present.

In certain embodiments, an increase of removal rate per increase of kinetic energy of abrasive particles is determined for abrasive jetting.

The removal rate can in particular be a volume removal rate, which can be calculated from the rate of penetration (depth per unit of time) times the cross sectional area of the borehole that is being drilled. (Velocity times cross sectional area gives volume per time.) Mass removal rate is less practical as it also includes the density of the rock which is another parameter that can complicate the analysis.

According to one aspect of the invention the at least one property of the formation material is a material property that is independent of a drilling or excavation method used for accessing the formation material.

According to another aspect of the invention, the at least one property of the formation material is a material property characteristic for the formation material independent of the position of the formation material in an earth formation. The material property is thus not a formation property, i.e. a not a property that is characteristic for a physical parameter in an, typically extended, subterranean formation, for example a formation pressure. Preferably the material property is characteristic for the material regardless whether it is in the formation or whether it has been removed from the earh formation to surface and ambient conditions. The material property is characteristic for a sample of the material of for example less than 1000 kg, or less than 100 kg, or less than 10 kg.

The invention will further be discussed and illustrated with reference to the graphs shown in the drawings, wherein

FIG. 1 shows a comparison of the clean water penetration rates in dependence of bit pressure drop for several types of rock material;

FIG. 2 shows typical jet drilling performance for sandstone at constant flow rate, with and without abrasives;

FIG. 3 shows typical jet drilling performance for sandstone at constant nozzle cross-section, with and without abrasives;

FIG. 4 shows jet drilling performance for sandstone at constant nozzle cross-section, for various concentration of abrasives;

FIG. 5 shows jet drilling performance with and without abrasives for several types of stone.

The invention is based on the insight that rock material reacts to the combined effects of a high pressure drilling fluid jet and abrasive particles in a very specific way, which depends on characteristic parameters such as permeability, porosity, tensile strength and the like. By comparing the removal rate, in particular volume removal rate of formation material in dependence on at least two settings of erosive power, it is possible to deduct one or more properties of the material being eroded by the jet. For instance, the volume removal rate of an abrasive jet depends on the particle concentration. It appears that the volume removal rate is proportional to said particle concentration, at least up to a certain concentration, which was e.g. found to be less than 4 wt % of steel shot in water. Also, the removal rate appears to be proportional to the hydraulic power applied.

As mentioned before, the porosity and the permeability of formation material may be established on the basis of said predetermined relationship. Also the tensile strength may be established, which properties once known all play a role in determining the formation material which is being drilled.

The predetermined relationship between the removal rate and the nozzle pressure drop of the high velocity jet, in the absence of abrasive particles, may be given in the form of a graph the horizontal abscissa of which represents the pressure drop and the vertical ordinate of which represents the volume removal rate, said graph furthermore comprising lines which are indicative of the relationship between these ordinates for particular formation materials, such as sandstone, limestone, marble and schist. Such graph may be presented in digital format in a logic evaluation device.

FIG. 1 shows the dependence of volume removal rate VRR (given in arbitrary units) of the nozzle pressure drop Δp (in bar) for different rock types; a) Glidenhaus sandstone (porosity por=25%, permeability perm=3000 mD); b) Euvile limestone (por=15%, perm=30 mD, c) Obernkirchen sandstone (por=19%, perm=5 mD), d) Carrara marble (por=1%, perm<0.1 mD), e) Martelange schist (perm<0.1 mD), e) Belgian limestone (por=1.5%, perm<0.1 mD, f) Solnhofen limestone (por=4%, perm<0.1 mD).

As shown in FIG. 1, independent of the presence of abrasives or their concentration, a high pressure water jet can blow up the pores if the rock is porous and permeable. The nozzle pressure drop that drives the jet should exceed a certain threshold pressure, dependent on rock type, before rock destruction by this process can occur. Beyond this threshold pressure the rock destruction is proportional to the nozzle pressure drop minus the pressure threshold P_t. Measurement of the volume removal rate (or rate of penetration) as a function of the flow rate (and no abrasive particles in the drilling fluid) gives two parameters: P_t and the volume removal rate per bar above P_t). This is also shown by the straight curves in the figure. The volume removal rate of a water jet without abrasive particles is proportional to the hydraulic power, which is determined by bit pressure drop. Doubling the hydraulic power of the jet above a certain threshold by doubling the bit pressure drop above a threshold leads to a doubling of the removal rate as well. The volume removal rate is defined as the penetration depth times the cross sectional area of the borehole per unit of time.

From FIG. 1 it is furthermore clear that for every type of rock a certain lower threshold exists beneath which the water jet does not have a noticeable effect on the formation material.

FIG. 2 shows Volume Removal Rate VRR in arbitrary units as a function of bit pressure drop in bar in a typical porous sandstone, and at constant water flow though the bit nozzle, for a) a high pressure fluid jet without abrasives; b) the effect of abrasives alone, and c) the combined effect of high pressure fluid jetting and abrasives. The threshold pressure P_t is indicated as well.

FIG. 3 shows Volume Removal Rate VRR in arbitrary units as a function of bit flow rate BFR of water in arbitrary units, for a typical porous sandstone, and at constant nozzle flow area (constant nozzle diameter), for a) a high pressure fluid jet without abrasives; b) the effect of abrasives alone, and c) the combined effect of high pressure fluid jetting and abrasives. The bit flow rate corresponding to the threshold pressure P_t is indicated as well. In a typical jet drilling operation, since the nozzle(s) is/are downhole, they cannot be changed and variation of the nozzle pressure drop goes together with varying the drilling fluid flow rate through the nozzle(s) in the bit.

FIG. 4 shows the influence of different abrasive particle concentrations on the curves c) from FIG. 3. Curve a) contains no particles (corresponding to curve a from FIG. 3), and shows the threshold pressure. Curves b), c), d), e) contain respectively 1, 2, 3 and 4 vol % abrasive particles (steel shot) content in water. The higher the abrasive particles content, the greater the removal rate is.

FIG. 5 shows a comparison of the volume removal rate for water without abrasives (curves A1, B1, C2) and with 2 vol % steel shot in water (curves A2, B2, C2), for different types of rock (A=soft porous sandstone, B=soft shale, C=hard tight rock), again as a function of bit flow rate BFR in arbitrary units and at constant nozzle flow area (constant nozzle diameter). It is observed that the high pressure liquid jet without abrasives has hardly any influence on the soft shale, which has no pores. The abrasive influence of the particles on the soft shale is large. The influence of the abrasive particles on hard tight rock is relatively small: the liquid jet has a limited influence due to the presence of cracks in the rock. Soft porous sandstone is readily eroded by the abrasive particles, and also there is some influence of the liquid jet.

Based on this behavior, several types of stones can be distinguished and/or parameters derived by studying the volume removal rate, or a parameter related thereto such as rate of penetration, as a function of erosive power (e.g. bit flow rate, bit pressure drop).

As can be seen from the Figures, and in contrast to conventional methods for drilling through rock with, for instance, tricone or PDC bits, the rock removal rate of high pressure water jetting depends strongly on parameters like permeability and porosity of the target rock type. In the case of a rock with very low permeability and porosity, shale for instance, a high-pressure jet without solids does not penetrate or hardly penetrates the rock. In the case of permeable and porous sandstones and limestones, a jet generated with a nozzle pressure drop higher than a certain threshold value, can easily penetrate. For comparison, shale is commonly considered a soft rock and relatively easy to drill using mechanical drills, and likewise there are also soft sandstones.

For a jet containing abrasive particles the dependence is different. For rock removal by abrasive particle impacts, there is no threshold value and soft rock is jetted away easier than hard rock. At the same time, there is the impact of the high velocity jet that may remove rock by penetrating the microcracks generated by the particle impacts, but also because of the already existing permeability and porosity of the rock. By varying erosive power such as by varying one or more of the bit pressure drop, the bit flow rate at constant nozzle area, the abrasive particle content of an abrasive jet drilling bit can distinguish e.g. between a soft impermeable formation material, a porous medium strength rock, or a hard and tight rock.

A single curve or a plurality of curves in one of the Figures, or equally well a mathematical expression describing the curve(s) can be regarded as a relationship between erosive power (on the abscissa of the curves) and a removal rate of formation material (on the ordinate of the curves). By determining removal rates for at least two settings, qualitative, semi-quantitative or quantitative information can be derived about one or more parameters of the formation material influencing the curves.

Optionally in combination with the fine cuttings typically produced by (abrasive) jet drilling bits, an estimate of the type of rock and its porosity, permeability, and tensile strength can be made. Rock formation materials will have a ‘fingerprint’ for the rate of penetration as a function of abrasive particle concentration and/or bit pressure drop.

Concerning the dependence from the abrasive particle concentration, as long as the bit pressure drop is below the pure liquid (e.g. clean water) threshold pressure, the volume removal rate of abrasive jet drilling bits is proportional to the hydraulic power and abrasives concentration at the bit. Hard rock is harder to drill than soft rock, but doubling the hydraulic power or the particle concentration in the jet(s) leads to twice the volume removal rate. Only beyond a certain abrasive particle concentration of typically 4% by volume (steel shot in water), the particles start losing some of their impact energy through intra-particle collisions and the volume removal rate is not proportional anymore to the abrasive particle concentration.

The abrasive particles destruct rock by their high velocity impact. The volume removal by this abrasive jet drilling (AJD) process is proportional to the kinetic energy in the abrasive particles, i.e. proportional to (abrasives concentration)*(flow rate)*(nozzle pressure drop), and there is not threshold. The amount of kinetic energy in the particles in the jet(s) can be varied by varying the hydraulic power that drives the jetstream(s) (at constant particle concentration) or by varying the amount of particles, i.e. the abrasive particle concentration) in the jet stream(s). This yields another parameter: the (volume removal rate per Watt kinetic power in the abrasive particles.

Concerning the dependence on the bit pressure drop, for a given constant abrasives mass flow, the rock removal rate is a rock and abrasives concentration dependent factor times the hydraulic power W=(Nozzle pressure drop)×(flow rate), at the bit. A high pressure abrasive jet can blow up the pores if the rock is porous and permeable. The nozzle pressure drop that drives the jet should exceed a certain threshold pressure before rock destruction by this process can occur. Beyond this pressure the rock destruction is proportional to the nozzle pressure drop minus the pressure threshold. Measurement of the volume removal rate (or rate of penetration) as a function of the flow rate (and no abrasive particles in the drilling fluid) gives two parameters: threshold pressure P_t and the (volume removal rate VRR per pressure difference above P_t).

The two processes mentioned above can yield three parameters of the relationship between erosive power and removal rate, that depend on three unknown properties: the porosity, the permeability and the tensile strength of the formation material (formation rock). This is illustrated in the Figures. Shale has negligible permeability and porosity: volume removal rate without particles is minimal. But shale is also relatively soft: the volume removal rate for abrasive jetting drilling AJD is typically relatively high. Porous sandstone or porous limestone can be jetted by high pressure jetting HPJ (without abrasives), giving a certain P_t pressure and HPJ volume removal rate. Sandstone is typically not very strong, so the volume removal rate by AJD can be relatively high. Through calibration curves the three measured parameters can be matched with the unknown porosity, permeability and tensile strength of the rock. The chemical composition of the rock cuttings can optionally still be derived from an analysis at surface, even if the cuttings are relatively small.

Tight hard rock (e.g. limestone, granite, bassalt) can have a low porosity and permeability and will have a relatively high threshold pressure. Similar to conventional, largely mechanical drilling processes, the AJD rock volume removal rate will be slower than for the weaker shales or sandstones.

Conducting a jet drill operation can be generally done as knows in the art. Suitable abrasive jet drill heads, systems and methods of operation are e.g. disclosed in WO 00/66872, WO 2002/034653, WO 2005/005766, WO 2005/005767, WO2008/119821, WO 2008/113843, WO 2008/113844, incorporated herein by reference. A recirculation system as for example described in WO 2002/034653, WO 2005/005766, WO2008/119821, WO 2008/113844 can be used, but this is not required. 

1. Method for determining a property of a formation material in the course of a jet drilling operation, wherein a fluid jet is blasted with erosive power into impingement with the formation material, the method comprising the steps of: postulating a relationship between the erosive power and a removal rate of formation material as a result of the impingement of the fluid jet with the erosive power on said formation material, the relationship comprising a parameter related to at least one property of the formation material; determining removal rates of formation material for at least two settings of erosive power; and determining the at least one property of the formation material from the relationship and the determined removal rates in dependence on erosive power.
 2. The method according to claim 1, wherein the fluid jet is an abrasive fluid jet comprising abrasive particles.
 3. The method according to claim 2, wherein the concentration of abrasive particles in the fluid jet is different between the at least two settings of erosive power.
 4. The method according to claim 1 wherein at least one of a fluid pressure, pressure drop over a jet nozzle, or velocity of jetted fluid, is different between the at least two settings of erosive power.
 5. The method according to claim 1 wherein the at least one property comprises a classification of hardness of the formation material.
 6. The method according to claim 1 wherein the at least one property comprises a rock type.
 7. The method according to claim 1 wherein the at least one property comprises a porosity.
 8. The method according to claim 1 wherein the at least one property comprises a permeability.
 9. The method according to claim 1 wherein the at least one property comprises a tensile strength.
 10. The method according to claim 1 wherein the relationship includes the erosive power of hydraulic liquid and the erosive power of abrasive particles, and wherein the at least two settings of erosive power are selected such that the effect of changing only erosive power of hydraulic liquid and/or only erosive power of abrasive particles can be determined individually.
 11. The method according to claim 1 wherein at least one of a threshold pressure and an increase of removal rate per increase of hydraulic pressure above a threshold pressure is determined for hydraulic liquid jetting.
 12. The method according to claim 1 wherein an increase of removal rate per increase of kinetic energy of abrasive particles is determined for abrasive jetting.
 13. The method according to claim 1 wherein the at least one property of the formation material is a property that is independent of a drilling or excavation method used for accessing the formation material.
 14. The method according to claim 1 wherein the at least one property of the formation material is a material property characteristic for the formation material independent of the position of the formation material in an earth formation. 